Hydrogen is a secondary energy source that is suitable for storage and transport, and has little environmental impact. As a result, hydrogen energy systems that use hydrogen as an energy carrier are attracting much interest. Currently, hydrogen is mainly produced by steam reforming of fossil fuels or the like. From the viewpoints of problems such as global warming and fossil fuel depletion, the importance of alkaline water electrolysis which uses renewable energy such as solar cells, wind power or hydroelectric power as a power source continues to increase.
Water electrolysis can be broadly classified into two types. One type is alkaline water electrolysis, which uses a high-concentration alkaline aqueous solution as the electrolyte. The other type is solid polymer water electrolysis, which uses a diamond electrode and uses a solid polymer electrolyte (SPE) as the electrolyte. When large-scale hydrogen production is performed by water electrolysis, it is considered that alkaline water electrolysis using an inexpensive material such as an iron-based metal of nickel or the like is more suitable than solid polymer water electrolysis using a diamond electrode.
The electrode reactions at the two electrodes are as follows.Anode reaction: 2OH−→H2O+½O2+2e−  (1)Cathode reaction: 2H2O+2e−→H2+2OH−  (2)
High-concentration alkaline aqueous solutions increase in conductivity as the temperature increases, but the corrosiveness also increases. Accordingly, the upper limit for the operating temperature is suppressed to about 80 to 90° C. The development of electrolyzer structural materials and various piping materials that are capable of withstanding higher temperatures and high-concentration alkaline aqueous solutions, and the development of low-resistance diaphragms and electrodes having increased surface area and provided with a catalyst have enabled electrolysis performance to be improved to about 1.7 to 1.9 V at a current density of 0.3 to 0.4 Acm−2 (efficiency: 78 to 87%).
The anode for alkaline water electrolysis typically uses a nickel-based material that is stable in the high-concentration alkaline aqueous solution, and it has been reported that a Ni-based electrode has a lifespan of several decades or longer in alkaline water electrolysis that uses a stable power source (Non-Patent Documents 1 and 2). However, Ni electrodes have a high overpotential, and suffer from poor productivity.
Conventionally, porous nickel or Ni or an alloy thereof has been used as the substrate of the anode for oxygen generation used in alkaline water electrolysis (Patent Document 1).
Further, the following types of metals or metal oxides have conventionally been used as the electrode catalyst layer of the anode for oxygen generation used in alkaline water electrolysis.
(1) Raney nickel (Patent Document 1)
(2) Platinum-group metals (Patent Documents 2 to 4)
Patent Document 2 discloses an anode that uses nickel and rhodium. Patent Document 3 discloses an anode that uses nickel, cobalt or silver, together with ruthenium, rhodium or iridium. Patent Document 4 discloses an electrode that uses platinum.
(3) Platinum-group metal oxides such as ruthenium oxide and iridium oxide (Patent Document 5)
(4) Alloys of a first metal composed of at least one metal selected from among iron, titanium, niobium, zirconium, tantalum, tin, molybdenum and bismuth, and a second metal composed of at least one metal selected from among nickel, cobalt, silver and platinum (Patent Document 6)
(5) Alloy systems based on nickel such as Ni—Co and Ni—Fe, nickel having an expanded surface area, and spinel Co3O4 and NiCo2O4 as ceramic materials (Patent Documents 7 and 8)
(6) Conductive oxides such as perovskite LaCoO3 and La0.6St0.4CoO3 (Patent Document 9)